Olefins are an important and versatile feedstock for fuels and chemicals, but they are not as widely available naturally as alkanes. The chemical industry uses olefins as intermediates in a variety of processes. The largest chemical use is linear α-olefins used in the formation of polyolefins such as ethylene-1-octene copolymers. Also and most importantly, low carbon number olefins have the potential to be converted into higher carbon number molecules that would be suitable for fuels, particularly, diesel. Other products formed from olefins include surfactants, lubricants, and plasticizers. Thus, the direct production of alkenes from alkanes via dehydrogenation has drawn great attention. Many heterogeneous catalysts are known to effect dehydrogenation at high temperatures (ca. 500-900° C.), but applications are limited to simple molecules such as ethane or ethylbenzene due to the low selectivity of these catalyst systems. In the case of higher alkanes, lack of selectivity (including catalyst-deactivating coking) severely impacts the utility of dehydrogenation.
Many iridium complexes as catalysts are known. During the 1980s, it was discovered that certain iridium complexes are capable of catalytically dehydrogenating alkanes to alkenes under thermal and photolytic conditions (see, e.g., J. Am. Chem. Soc. 104 (1982) 107; 109 (1987) 8025; J. Chem. Soc., Chem. Commun. (1985) 1829). For a more recent example, see Organometallics 15 (1996) 1532.
Pincer ligand complexes of rhodium and iridium as catalysts for the dehydrogenation of alkanes are receiving widespread attention. See, for example, F. Liu, E. Pak, B. Singh, C. M. Jensen and A. S. Goldman, “Dehydrogenation of n-Alkanes Catalyzed by Iridium “Pincer” Complexes: Regioselective Formation of α-olefins,” J. Am. Chem. Soc. 1999, 121, 4086-4087; F. Liu and A. S. Goldman, “Efficient thermochemical alkane dehydrogenation and isomerization catalyzed by an iridium pincer complex,” Chem. Comm. 1999, 655-656; and C. M. Jensen, “Iridium PCP pincer complexes: highly active and robust catalysts for novel homogenous aliphatic dehydrogenations,” Chem. Comm. 1999, 2443-2449. The use of compounds such as (PCP)MH2 (PCP=C6H3(CH2PBut2)2-2,6) (M=Rh, Ir) dehydrogenate various cycloalkanes to cycloalkenes with turnovers of 70-80 turnovers/hour.
Various pincer catalysts supported on solid supports via polar anchoring groups are also known. See Huang, Z.; Brookhart, M.; Goldman, A. S.; Kundu, S.; Ray, A.; Scott, S. L.; Vicente, B. C. Adv. Synth. Catal. 2009, 351, 188 and Huang, Z.; Rolfe, E.; Carson, E. C.; Brookhart, M.; Goldman, A. S.; El-Khalafy, S. H.; MacArthur, A. H. R. Adv. Synth. Catal. 2010, 352, 125.
In addition, “pincer” complexes of platinum-group metals have been known since the late 1970s (see, e.g., J. Chem. Soc., Dalton Trans. (1976) 1020). Pincer complexes have a metal center and a pincer skeleton. The pincer skeleton is a tridentate ligand that generally coordinates with the meridional geometry. The use of pincer complexes in organic synthesis, including their use as alkane dehydrogenation catalysts, was developed during the 1990s and is the subject of two review articles (see Angew. Chem. Int. Ed. 40 (2001) 3751 and Tetrahedron 59 (2003)). See also U.S. Pat. No. 5,780,701. Jensen et al. (Chem. Commun. 1997 461) used iridium pincer complexes to dehydrogenate ethylbenzene to styrene. Recently, additional pincer complexes have been developed that dehydrogenate hydrocarbons. For some recent examples, see J. Mol. Catal. A 189 (2002) 95, 111 and Chem. Commun. (1999) 2443.
Initial attempts to design effective homogeneous catalytic systems for alkane dehydrogenation were hampered by catalyst decomposition. In subsequent attempts, Kaska and Jensen reported the first ever robust system for catalytic transfer dehydrogenation of alkanes that was based on a pincer-ligated iridium complex (tBu4PCP)Ir(H2). (See, Gupta, M., Hagen, C., Flesher, R. Chem. Commun. 1996, 36, 2083-2084 and Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.; Jensen, C. M.; Barbara, S. J. Am. Chem. Soc. 1997, 267, 840-841.) Later it was observed that both (tBu4PCP)Ir(H2) and its isopropyl analog (iPr4PCP)Ir(H4) were found to selectively activate C—H bond of alkanes. (See, Liu, F., Pak, E. B., Singh, B., Jensen, C. M., Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087.) These catalysts were also effective for the acceptorless dehydrogenation of alkanes. (See, Xu, W., Rosini, G. P., Krogh-Jespersen, K., Goldman, A. S., Gupta, M.; Jensen, C. M.; Kaska, W. C. Chem. Commun. 1997, 2273-2274; and Liu, F.; Goldman, A. S. Chem. Commun. 1999, 655-656; and Krogh-Jespersen, K.; Czerw, M.; Summa, N.; Renkema, K. B.; Achord, P. D.; Goldman, A. S. J. Am. Chem. Soc. 2002, 124, 11404-16.) These studies paved the way for design of several pincer based catalytic systems for alkane dehydrogenation reactions and their mechanistic studies. (See, Renkema, K. B.; Kissin, Y. V; Goldman, A. S. J. Am. Chem. Soc. 2003, 125, 7770-1; and Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761-79; and Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Acc. chem. res. 2012, 45, 947-58.) Over the years many research groups have reported variants of (tBu4PCP)Ir(H2) and (iPr4PCP)Ir(H4) where either the substituent at the para position of the aryl group has been altered (see Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2004, 126, 13044-53; and Huang, Z.; Brookhart, M.; Goldman, A. S.; Kundu, S.; Ray, A.; Scott, S. L.; Vicente, B. C. Adv. Synth. Catal. 2009, 351, 188-206) or the CH2 linkers have been modified. (See, Gottker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330-8; and Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804-11; and White, P. S.; Brookhart, M.; Hill, C.; Carolina, N. Organometallics 2004, 23, 1766-1776; and Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.; Brookhart, M.; Goldman, A. S. Nat. chem. 2011, 3, 167-71; and Dobereiner, G. E.; Yuan, J.; Schrock, R. R.; Goldman, A. S.; Hackenberg, J. D. J. Am. Chem. Soc. 2013, 135, 12572-5; and Shi, Y.; Suguri, T.; Dohi, C.; Yamada, H.; Kojima, S.; Yamamoto, Y. Chem. Eur. J. 2013, 19, 10672-89.) Such phosphine, phosphinite and mixed phosphine-phosphinite based systems have found wide spread utility as catalysts for alkane metathesis (see, Haiback, M. C. Kundu, S.; Brookhart, M.; Goldman, A. S. Acc. chem. res. 2012, 45, 947-58; and Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312, 257-61; and Ahuja, R.; Kundu, S.; Goldman, A. S.; Brookhart, M.; Vicente, B. C.; Scott, S. L. Chem. Commun. 2008, 253-5), alkyl group metathesis (see, Dobereiner, G. E., Yuan, J.; Schrock, R. R.; Goldman, A. S.; Hackenberg, J. D. J. Am. Chem. Soc. 2013, 135, 12572-5), dehydroaromatization reactions (see, Ahuja, R., Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.; Brookhart, M.; Goldman, A. S. Nat. chem. 2011, 3, 167-71), alkane-alkene coupling reactions (see, Leitch, D. C.; Lam, Y. C.; Labinger, J. A; Bercaw, J. E. J. Am. Chem. Soc. 2013, 135, 10302-5) and dehydrogenation of several other substrates. (See, Gupta, M.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1997, 461-462; and Jensen, C. M. Chem. Commun. 1999, 2443-2449; and Zhang, X.; Fried, A.; Knapp, S.; Goldman, A. S. Chem. Commun. 2003, 2060-1.) Recently, there has also been an attempt to understand the steric effects on catalytic efficiency by systematically replacing the phosphino-tert-butyl groups with phosphino methyl groups. (See, Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.; Warmuth, R.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S. Organometallics 2009, 28, 5432-5444.)
One of the widely used hydrogen acceptors for alkane transfer dehydrogenation is tert-butyl ethylene (TBE) as it is resistant to isomerization reactions. However on an industrial scale, the use of TBE is less economical. One would prefer to use hydrogen acceptors that are inexpensive and recyclable.
Despite the extensive research into new catalysts and methods for producing valuable olefin compounds, the search for effective methods to prepare olefins from alkanes continues. Such methods would make the preparation of valuable olefin compounds more economical and efficient.